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How to Battle Mother Nature

Mold on hardscaping can be avoided with simple techniques!

Hardscaping can be relatively maintenance-free.  However, issues may arise if your patio or pool deck is not properly maintained or the initial installation isn’t correct.  The first topic in the “Battling Mother Nature” series addresses how to avoid having a persistent mold and/or algae issue on any type of hardscape installation.

The problem is often thought to be not enough sunlight in that area (particularly in the case of mold and mildew).  Of course, it may help to have plenty of sunlight to mitigate the mold growth on the hardscaping. However, the problem is not the surface. It lies underneath.  The problem is almost always about the substrate in paver, tile and other types of exterior or interior environments.   The unfortunate appearance of mold and/or algae can readily be avoided with proper installation vs the age-old excuse of insufficient sunlight or other natural causes. 

The Problem and Solutions  

So, how is it that two adjoining properties, with similar environmental circumstances and conditions, yield two different outcomes: one has a serious mold and/or algae problem while the other property has no issues whatsoever?   

Peach Yellow Travertine mold problems

The problem occurs when algae and/or mold and other fungal types grow on damp or moisture laden pavers.   The answer is to eliminate the presence of moisture from underneath. Without moisture, the algae/mold cannot survive.  The key is proper installation with the substrate being the most important component.

People that do have these ongoing nightmarish issues, are having these problems because the preparation, prior to the installation of the pavers, was not done properly.  Yes, maybe they installed 12 – 18” of gravel underneath, and they did an inch of sand on top of the gravel, and they used geotextile (which hardly anyone uses) and all that is well and good. However, Mrs. Smith, has that problem, and Mrs. Jones, next door, doesn’t. 

One reason can be the pooling of the water.  Another contributing factor can be the use of “modified” gravel, which inhibits the ease of movement for water to drain properly, instead of clean/screened gravel.  Water will always seek out the lowest point.  It rains and physics says that gravity will pull the water to the lowest point.  When there isn’t a place for water to escape, it ends up sitting underneath the installation, like a pond or, in the case of a pool deck, like a moat circling the pool and the water has no place to go.  So, what happens?  The water just sits there and feeds these microorganisms that end up being mold growths and discoloration on your beautiful pavers.  Look familiar?   

Most important is to be informed on how to properly integrate an escape route for the water. These types of dry-set hardscaping projects always involve excavation and a new sub-base of compacted gravel (hopefully clean/washed gravel).  When you have a wet-set (concrete slab) installation, the mold, typically, does not grow up and around the entire slab, and, therefore, the mold issue is not as prevalent with slab projects.   You may experience some mold around the corners or edges of the slab but, mold typically doesn’t grow through the slab. 

A proper French drain, when your patio or pool deck is installed, is the answer.  It’s all good to have the gravel and the sand but the water needs to have a place to go.  When digging that moat around the pool, or excavating underneath the proposed patio, be sure the water that goes down to that gravel is routed away.  If it doesn’t have some place to go, you will, likely, have this type of moisture problem. 

Some types of soil “percolate” so well that the water will just drain.  Poor percolation is an intrinsic problem in clay soils, however.  Soils that percolate better, unlike clay soils, let the water escape through the soil.  And, that is quite common….however, best not to risk it.  You want to make sure that the water has some place to go whether you have clay soil or not.  When there is a strong grade on the property, water will likely find its way downhill.  And, if your deck or patio is built up above the grade, then water, typically, will seep out and go downhill somewhere.  No matter the situation, you want to have a built-in-exit route for the water, even if it is at only one point.   

Example of a recommended French drain

Courtesy of Home Improvement Stack Exchange

Existing Patios or Pool Decks? 

Sure, you can power wash over and over but that is just treating the symptom and not treating the cause.  So, what do you do?  Unfortunately, for many people who are reading this, you might be past the installation phase without the proper drainage pipes.  

Here’s what to do.  

  • Pull up as many pavers as you need to in order to find the lowest spot, in the original excavation, and utilize some method to help the water escape.
  • Replace some of the gravel and underneath that gravel bed, place PVC pipes, to allow that water to escape the pool deck or patio area and to drain out to a lower point. 
  • Go to the lowest point of your excavation and that’s where the water will be accumulating. 
  • Use 1” PVC pipe with holes drilled throughout  (of course the bigger the pipe the better, but this does less collateral damage, particularly in a retrofit situation) and run it to either a water collection area where you may have a sump pump or let it percolate through your yard.
Beautiful Peach Yellow Travertine patio project recently renovated

Point of Interest:  “Modified” vs Clean/Screened Gravel 

If you use clean, screened gravel, which is what we strongly recommend.  Unfortunately, most contractors use modified gravel and it is not clean/screened gravel.  It compacts very nicely and the ICPI guidelines recommend this modified type of material vs cleaned gravel.  We believe the gravel industry pushes it very hard because it is very easy for them to produce; less than half the work for them to do; most of the work in making gravel is grading (sizing) and cleaning it.  The gravel quarry industry loves selling the modified.  The installers love it because it compacts very easily. Clean or screened gravel, on the other hand, is tricky because it locks into place – the different facets or faces of the gravel lock into each other and it is more difficult to compact, it is more work.  But   dry-set or wet-set, because it makes a barrier to the dirt that isn’t excavated. Specs for doing roads (and most concrete installations) specify to use washed, screened or cleaned gravel (not modified) in conjunction with geotextile. 

Maintenance Tips for pool decks or patios

Hardwood decks/patios as well as wood on houses/buildings need to be power washed, cleaned and sealed on a regular basis.   Although you power wash and seal a hardwood deck every year, natural stone deck maintenance is much less frequent.  We recommend casual cleaning and sealing, with the proper sealers, at least every several years depending on what sealer product you use.  It’s important to note, that you should not use a prophylactic coating type of sealer like you would on wood products.  That type of sealer puts a coating on the wood and people often use that kind of sealer for concrete pavers, as well, with bad results.  With natural stone, you should never use a prophylactic coating type of sealer because the stone wants to breathe.  And, that is a good thing.  Use a “breathable” sealer which tend to last many years longer.  When using a high-quality sealer, you will only have to seal every 5-10 years. 

How to get stone to visually “pop” again like it was new

We like to recommend that every couple of years, you do a mild muriatic acid wash on the stone.  You must be careful but, it is not as scary as you may think.

Here’s how:
  • Saturate the surface of the stone, where you are working, with water. 
  • Use a 10-20% dilution of muriatic acid that you would purchase from your pool supply store or hardware store (it is already diluted when you purchase the container).  
  • Take the diluted jug and dilute it again to 5-1 (5 parts water, 1-part of the purchased diluted muriatic acid).  
  • Wash your deck/patio stone with the twice diluted muriatic acid (you can use a mop or bug sprayer).
  • Sweep with a broom, then rinse it off with a heavy water flush.  

Note:  The acid should not be strong enough to do any damage to the ecology or be harmful to your flower beds next to the stone patio because once it makes contact with the stone, there is a little bit of sizzle, a bit of chemical reaction that happens and then it should be neutralized. 

We hope you have found these tips useful – please share with other contractors and homeowners! Thank You, The Stonehunter.

Gothicstone Consulting Services


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